1. Field of the Invention
The present invention relates to a plated steel sheet used for cans such as food cans and, more particularly, to a plated steel sheet suited to food cans adopting an aluminum easy-open top.
2. Description of the Prior Art
Tin-plated steel sheet, tin free steel (obtained by forming a chromium plating layer on the surface of a steel sheet and forming a hydrated chromium oxide layer thereon), and aluminum plates have been conventionally widely used as can materials. As easy-open tops are increasingly used for drink cans, full-open end cans adopting an aluminum easy-open top have been recently used for food cans. Easy-open cans of this type can be conveniently opened without a can opener and therefore are strongly demanded. For this reason, a demand has arisen for supply of inexpensive and reliable can materials.
Conventionally, both a can top and a can body of a full-open end can are made of aluminum. Aluminum is, however, more expensive than a tin-plated steel sheet or a chromium-plated steel sheet, and its strength is unsatisfactory. Therefore, aluminum is damaged during handling, or defective cans are sometimes produced. In addition, although aluminum has a good corrosion resistance to general food, its corrosion resistance to highly corrosive can contents containing a large amount of salt such as salted food or food cooked with soy sauce is not satisfactorily reliable.
In consideration of the above situation, a method has been proposed in which properties of both aluminum and steel are utilized, i.e., soft aluminum is used as an easy-open top and a surface-treated steel sheet having strength and an under film corrosion resistance is used as a can body which must have strength so that a corrosion resistance against a can content is obtained by a paint coated on the inner surface of the can. One of a can body and a can top made of different materials is selectively dissolved and corroded, i.e., a problem of so-called bimetallic corrosion is posed. The bimetallic corrosion is a phenomenon in which when two types of metals having different electrode potentials are placed in the presence of an electrolyte and are electrically brought into contact with each other, both the metals serve as electrodes to form a cell, a current flows between the metals from a relatively noble one to a base one through a contact point therebetween, and the base metal is ionized and dissolved. When a can top is made of aluminum and a can body is made of a tin-plated steel sheet, aluminum serves as a base metal and tin serves as a noble metal. Therefore, aluminum is ionized by an anode reaction, and hydrogen is produced on the surface of tin plating by a cathode reaction. If the aluminum top has a film defect, this defect portion is locally dissolved, and a hole is produced by pitting. At the same time, a film on the tin plating is peeled by hydrogen produced at the cathode to corrode the tin-plated steel sheet. This phenomenon similarly occurs in tin free steel. Especially when chlorine ions are contained in a can content, the aluminum top turns to a base metal more easily, and the phenomenon occurs more significantly.
In order to prevent such bimetallic corrosion, a method of increasing the strength of a film coated on the inner surface of a can is studied, but a cost is inevitably increased in this method. In addition, a method is studied in which a potential behavior of an aluminum top is examined to make some improvements in an aluminum alloy designing step (see, for example, "Iron and Steel", 1987, Vol. 3, PP. 427 to 436). This method is, however, not practically used yet.
Aluminum can be plated on a steel sheet by conventional techniques. Examples of the conventional techniques are a method of manufacturing an aluminum single layer-plated steel sheet utilizing vapour deposition (Japanese Patent Publication Nos. 45-5123, 45-19762, 46-39445 and 59-32544) and a method of manufacturing a steel sheet having different metals, i.e., aluminum as an upper layer and Ti, Cr or Zn as a lower layer formed thereon (Japanese Patent Publication Nos. 46-4047, 46-25608 and 46-42006). Both of these methods, however, aim at improving a corrosion resistance of a steel sheet such as resistance to sprayed salt water but do not aim at using such a plate as a can material. Therefore, in these methods, an under film corrosion resistance is not taken into consideration at all.
As described above, an aluminum-plated steel sheet aiming at improving a general corrosion resistance to serve as a can body material of a convenient full-open can have a problem of an under film corrosion resistance. On the other hand, a tin-plated steel sheet or tin free steel as a conventional can material having an under film corrosion resistance poses a problem of bimetallic corrosion.